I don't understand your explanation for array {1,2,3,4,5,6} since the {1,100,50,-51,1,1} array has a solution with index 1. What is the difference between these arrays ?

Thank you, it's pretty clear

Means that you don't have to transform the characters that are not alphabetic or digits: transform a-z, A-Z, 0-9 but not &,!,$,* etc...
Example, in:
Test.assertEquals(playPass("I LOVE YOU!!!", 1), "!!!vPz fWpM J")
you can see that "!!!" is kept without transformation.
Tell me if it is not clear enough.

I'm almost there, but I don't understand the third task: keep such as non alphabetic and non digit characters. I'm waiting for explanation. Thank you in advance.

Thank you for the explanation, I got it.

If the answer was 3 for this kata, that would mean that the numbers on either side of index 3 are equal. So in this case, index 3 is equal to the number 4. This would mean that the numbers to the left of 4 (1+2+3) are equal to the numbers to the right of 4 (5+6). This is not the case because 6 != 11. If you are still confused after this explanation, let me know what you are having trouble understanding.

Oo =)